Related Field
The present disclosure relates to an antenna system for transmission and reception of polarized signals, and in particular to transmission and reception of polarized radar signals.
Description of Related Art
Low frequency radar systems, i.e., involving wavelengths on the order of meters, and airborne low frequency radar systems in particular, can be used for finding targets buried under ground or hidden below camouflage or trees. Low frequency radar systems can also be applied in establishing environmental parameters such as biomass.
The physics governing how low frequency radar signals interact with the ground depend to a high degree on the polarization state of the signal. Therefore, collecting data for both vertical and horizontal polarization is often valuable and sometimes even necessary.
Due to the comparably low frequencies involved, and the dependency on polarization state of the radar signal, the design of radar antennas in the area of airborne meter wavelength radar contains several significant challenges.
For instance, antennas must be physically quite large—the smallest high efficiency antenna is half a wavelength dipole, meaning that such dipoles will be of meter size. A dipole is not directive in contrast to conventional radar antennas, which extends for many wavelengths not only in one dimension, as a dipole, but in two dimensions. Just scaling such antennas is clearly not feasible for low frequency radar. Also, dipoles are often required to be wideband in the sense that the radar often needs to function and keep a reasonably constant antenna diagram or antenna pattern across a bandwidth of at least octave order.
Also, the polarization state of transmitted and received radar signals must often be controllable or selectable. In some applications, the polarization state also needs to be alternated with kHz order switching frequency, or used in parallel for horizontal and vertical polarization, so that radar response for both polarizations can be collected.
Hence, there is a need for an antenna system for use with low frequency radar systems which is comparably compact in terms of size, and where the polarization state of the transmitted and received radar signals can be controlled.